1. Field of the Invention
The present invention relates to a production apparatus for a fluoride crystal preferable for various kinds of optical elements, lenses, window materials, prisms, and the like to be used in a wide wavelength range from a vacuum ultraviolet region to a far-infrared region, and in particular, to a production apparatus for a fluoride crystal to be used for an optical material such as a large aperture lens (aperture of 250 mm or more) for an excimer laser.
2. Description of the Related Art
An excimer laser attracts attention as the only high-output laser that oscillates outside the ultraviolet region, so that widespread application thereof is expected in the electronics industry, chemical industry, and energy industry.
Specifically, the excimer laser is used in processing metal, resin, glass, ceramics and semiconductors, and in chemical reactions.
An apparatus for generating an exciser laser beam is known as an excimer laser oscillating apparatus. A laser gas filling a chamber, such as Ar, Kr, Xe, F2 or Cl2 is excited by electron beam radiation or by electric discharge. The excited atoms bond with atoms in the ground state so as to produce molecules-capable of existing only in the excited state. These molecules are the so-called “excimer”. Due to its instability, the excimer immediately discharges an ultraviolet ray and falls into the ground state. This phenomenon is called the bond free transition. An apparatus for taking out a laser beam by amplifying the ultraviolet ray obtained by this transition in an optical resonator comprising a pair of mirrors, is an excimer oscillating appartus.
Among excimer laser beams, a KrF laser and an ArF laser have a wavelength of 248 nm and 193 nm, respectively, in a vacuum ultraviolet ray region so that an optical system having a high transmissivity with respect to the wavelength region needs to be used. Examples of glass materials preferably used in the optical system include fluorides, such as calcium fluoride, magnesium fluoride, barium fluoride, neodymium fluoride, lithium fluoride, and lanthanum fluoride.
Hereinafter a conventional production method for a fluoride crystal will be explained with reference to an example of calcium fluoride called fluorite, which can be represented by the stoichiometric ratio of CaF2.
As conventional production methods for a fluoride crystal, methods disclosed in the official gasettes of Japanese Patent Laid-Open Nos. 4-349199 and 4-349198 can be presented. In short, in order to prevent the loss of weight in directly melting a high purity powdery material produced by chemical synthesis due to the bulk specific gravity, the high purity material in a cullet state is used, and is placed in a crystal growth furnace. Now the knowledge obtained by the present inventor to lead to the present invention will be described.
FIG. 11 is a schematic diagram showing a production method for a fluoride crystal initially conducted by the present inventor.
In the process S1, a powdery material is prepared. In the process S2, the material is placed in a container, melted, and then cooled. In the process S3, solidified agglomerates are pulverized with a stainless steel pulverizer. In the post-process S4, a fluorite block is produced by melting and gradually cooling the pulverized agglomerates placed in a crucible for crystal growth.
The process S2 is conducted for reducing the bulk specific density change before and after melting in the process S4, and further, for eliminating impurities from the material.
In the processes S2 and S4, a scavenger, which is a fluoride of a metal, is added in order to eliminate CaO produced by the reaction between the material (CaF2) and water, and the like, or impurities originally existing in the material. For example, a ZnF2 scavenger reacts with CaO to produce ZnO, and is eliminated at the time of melting the crystals. As a result, the CaO impurities are eliminated, so that a fluoride crystal having an excellent transmissivity characteristic can be obtained.
The fluoride crystal block accordingly obtained is cut in a desired thickness, processed and shaped into a desired lens shape to be used as an optical material.
In the discussion of the production conditions for obtaining a fluoride crystal having a still higher transmissivity by the present inventor, it was learned that the crucible structure exerted a great influence on the optical characteristics of the crystal after growth.
That is, in the study on the relationship between the conventional crucible structure to be used in the refining process and the transmissivity of the crystal that is finally obtained, it was learned that although the amount is slight, impurities in a material and a reaction product of a scavenger therewith cannot be wholly eliminated to the outside but in some amount remain inside the crucible, depending on crucible shape. The impurities deteriorate the optical characteristics of the final crystal, and generate irregularity in the optical parts made from that crystal.
In the crystal growth process, a refined block needs to be pulverized and placed in a crucible. However, since the bulk density is lowered by the pulverization, a larger crucible is required for obtaining a given desired crystal. Furthermore, due to the necessity of the pulverizing process, the productivity becomes poor. Moreover, a problem is involved in that a minute amount of impurities included at the time of the pulverization deteriorates the transmissivity of the crystal.
The present invention has been completed based on the foregoing knowledge obtained by elaborate study, and on still further elaborate study directed to the solution of the described problems. An object of the present invention is to provide a production apparatus for a fluoride crystal capable of producing a fluoride crystal having a high transmissivity, and efficiently eliminating such amounts of impurities and of scavenger materials remaining in the crystal.
Another object for the present invention is to provide a production apparatus of a fluoride crystal with a high productivity.